Error SLF4J: class loader has different class objects for type

Question

trying to figure out why I get the following error in tomcat logs:

Caused by: java.lang.LinkageError: loader constraint violation: when resolving method "org.slf4j.impl.StaticLoggerBinder.getLoggerFactory()Lorg/slf4j/ILoggerFactory;" the class loader (instance of org/apache/catalina/loader/WebappClassLoader) of the current class, org/slf4j/LoggerFactory, and the class loader (instance of sun/misc/Launcher$AppClassLoader) for the method defining class, org/slf4j/impl/StaticLoggerBinder, have different Class objects for the type org/slf4j/ILoggerFactory used in the signature 

In my gradle files I use slf4j version 1.7.7, and in tomcat I have jul-to-slf4j-1.7.7.jar and slf4j-api-1.7.7.jar, so I'm pretty confused.

Here is the stack trace - thanks for any help!

 Total time: 2.982 secs Using CATALINA_BASE: /usr/local/Cellar/tomcat/8.0.18/libexec Using CATALINA_HOME: /usr/local/Cellar/tomcat/8.0.18/libexec Using CATALINA_TMPDIR: /usr/local/Cellar/tomcat/8.0.18/libexec/temp Using JRE_HOME: /Library/Java/JavaVirtualMachines/jdk1.8.0_25.jdk/Contents/Home Using CLASSPATH: /usr/local/Cellar/tomcat/8.0.18/libexec/bin/jul-to-slf4j-1.7.7.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/slf4j-api-1.7.7.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/logback-classic-1.1.2.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/logback-core-1.1.2.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/logback-config/:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/bootstrap.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/tomcat-juli.jar 19:03:55.979 [main] INFO org.apache.catalina.startup.VersionLoggerListener - Server version: Apache Tomcat/8.0.18 19:03:55.981 [main] INFO org.apache.catalina.startup.VersionLoggerListener - Server built: Jan 23 2015 11:56:07 UTC 19:03:55.981 [main] INFO org.apache.catalina.startup.VersionLoggerListener - Server number: 8.0.18.0 19:03:55.982 [main] INFO org.apache.catalina.startup.VersionLoggerListener - OS Name: Mac OS X 19:03:55.982 [main] INFO org.apache.catalina.startup.VersionLoggerListener - OS Version: 10.10 19:03:55.982 [main] INFO org.apache.catalina.startup.VersionLoggerListener - Architecture: x86_64 19:03:55.982 [main] INFO org.apache.catalina.startup.VersionLoggerListener - Java Home: /Library/Java/JavaVirtualMachines/jdk1.8.0_25.jdk/Contents/Home/jre 19:03:55.982 [main] INFO org.apache.catalina.startup.VersionLoggerListener - JVM Version: 1.8.0_25-b17 19:03:55.982 [main] INFO org.apache.catalina.startup.VersionLoggerListener - JVM Vendor: Oracle Corporation 19:03:55.982 [main] INFO org.apache.catalina.startup.VersionLoggerListener - CATALINA_BASE: /usr/local/Cellar/tomcat/8.0.18/libexec 19:03:55.983 [main] INFO org.apache.catalina.startup.VersionLoggerListener - CATALINA_HOME: /usr/local/Cellar/tomcat/8.0.18/libexec 19:03:55.983 [main] INFO org.apache.catalina.startup.VersionLoggerListener - Command line argument: -Djava.util.logging.config.file=/usr/local/Cellar/tomcat/8.0.18/libexec/conf/logging.properties 19:03:55.983 [main] INFO org.apache.catalina.startup.VersionLoggerListener - Command line argument: -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager 19:03:55.983 [main] INFO org.apache.catalina.startup.VersionLoggerListener - Command line argument: -Djava.endorsed.dirs=/usr/local/Cellar/tomcat/8.0.18/libexec/endorsed 19:03:55.983 [main] INFO org.apache.catalina.startup.VersionLoggerListener - Command line argument: -Dcatalina.base=/usr/local/Cellar/tomcat/8.0.18/libexec 19:03:55.984 [main] INFO org.apache.catalina.startup.VersionLoggerListener - Command line argument: -Dcatalina.home=/usr/local/Cellar/tomcat/8.0.18/libexec 19:03:55.984 [main] INFO org.apache.catalina.startup.VersionLoggerListener - Command line argument: -Djava.io.tmpdir=/usr/local/Cellar/tomcat/8.0.18/libexec/temp 19:03:55.984 [main] INFO org.apache.catalina.core.AprLifecycleListener - The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: /Users/jadekler/Library/Java/Extensions:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java:. 19:03:56.076 [main] INFO org.apache.coyote.http11.Http11NioProtocol - Initializing ProtocolHandler ["http-nio-8080"] 19:03:56.092 [main] INFO org.apache.tomcat.util.net.NioSelectorPool - Using a shared selector for servlet write/read 19:03:56.094 [main] INFO org.apache.coyote.ajp.AjpNioProtocol - Initializing ProtocolHandler ["ajp-nio-8009"] 19:03:56.096 [main] INFO org.apache.tomcat.util.net.NioSelectorPool - Using a shared selector for servlet write/read 19:03:56.096 [main] INFO org.apache.catalina.startup.Catalina - Initialization processed in 510 ms 19:03:56.115 [main] INFO org.apache.catalina.core.StandardService - Starting service Catalina 19:03:56.115 [main] INFO org.apache.catalina.core.StandardEngine - Starting Servlet Engine: Apache Tomcat/8.0.18 19:03:56.132 [localhost-startStop-1] INFO org.apache.catalina.startup.HostConfig - Deploying web application archive /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/websiteskeleton.war 19:03:56.812 [localhost-startStop-1] ERROR org.apache.catalina.core.StandardContext - Error during ServletContainerInitializer processing javax.servlet.ServletException: Failed to instantiate WebApplicationInitializer class at org.springframework.web.SpringServletContainerInitializer.onStartup(SpringServletContainerInitializer.java:160) ~[spring-web-4.1.0.RELEASE.jar:4.1.0.RELEASE] at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5143) ~[catalina.jar:8.0.18] at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150) [catalina.jar:8.0.18] at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:725) [catalina.jar:8.0.18] at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:701) [catalina.jar:8.0.18] at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:714) [catalina.jar:8.0.18] at org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:917) [catalina.jar:8.0.18] at org.apache.catalina.startup.HostConfig$DeployWar.run(HostConfig.java:1701) [catalina.jar:8.0.18] at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [na:1.8.0_25] at java.util.concurrent.FutureTask.run(FutureTask.java:266) [na:1.8.0_25] at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_25] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_25] at java.lang.Thread.run(Thread.java:745) [na:1.8.0_25] Caused by: java.lang.LinkageError: loader constraint violation: when resolving method "org.slf4j.impl.StaticLoggerBinder.getLoggerFactory()Lorg/slf4j/ILoggerFactory;" the class loader (instance of org/apache/catalina/loader/WebappClassLoader) of the current class, org/slf4j/LoggerFactory, and the class loader (instance of sun/misc/Launcher$AppClassLoader) for the method defining class, org/slf4j/impl/StaticLoggerBinder, have different Class objects for the type org/slf4j/ILoggerFactory used in the signature at org.slf4j.LoggerFactory.getILoggerFactory(LoggerFactory.java:306) ~[slf4j-api-1.7.7.jar:1.7.7] at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:276) ~[slf4j-api-1.7.7.jar:1.7.7] at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:288) ~[slf4j-api-1.7.7.jar:1.7.7] at com.websiteskeleton.core.HelloInitializer.<init>(HelloInitializer.java:13) ~[HelloInitializer.class:na] at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_25] at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_25] at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_25] at java.lang.reflect.Constructor.newInstance(Constructor.java:408) ~[na:1.8.0_25] at java.lang.Class.newInstance(Class.java:438) ~[na:1.8.0_25] at org.springframework.web.SpringServletContainerInitializer.onStartup(SpringServletContainerInitializer.java:157) ~[spring-web-4.1.0.RELEASE.jar:4.1.0.RELEASE] ... 12 common frames omitted 19:03:56.821 [localhost-startStop-1] ERROR org.apache.catalina.core.StandardContext - Context [/websiteskeleton] startup failed due to previous errors 19:03:56.825 [localhost-startStop-1] INFO org.apache.catalina.startup.HostConfig - Deployment of web application archive /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/websiteskeleton.war has finished in 693 ms 19:03:56.826 [localhost-startStop-1] INFO org.apache.catalina.startup.HostConfig - Deploying web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/docs 19:03:56.893 [localhost-startStop-1] INFO org.apache.jasper.servlet.TldScanner - At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time. 19:03:56.904 [localhost-startStop-1] INFO org.apache.catalina.startup.HostConfig - Deployment of web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/docs has finished in 78 ms 19:03:56.904 [localhost-startStop-1] INFO org.apache.catalina.startup.HostConfig - Deploying web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/examples 19:03:57.133 [localhost-startStop-1] INFO org.apache.catalina.startup.HostConfig - Deployment of web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/examples has finished in 228 ms 19:03:57.133 [localhost-startStop-1] INFO org.apache.catalina.startup.HostConfig - Deploying web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/host-manager 19:03:57.192 [localhost-startStop-1] INFO org.apache.jasper.servlet.TldScanner - At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time. 19:03:57.195 [localhost-startStop-1] INFO org.apache.catalina.startup.HostConfig - Deployment of web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/host-manager has finished in 62 ms 19:03:57.195 [localhost-startStop-1] INFO org.apache.catalina.startup.HostConfig - Deploying web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/manager 19:03:57.252 [localhost-startStop-1] INFO org.apache.jasper.servlet.TldScanner - At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time. 19:03:57.254 [localhost-startStop-1] INFO org.apache.catalina.startup.HostConfig - Deployment of web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/manager has finished in 59 ms 19:03:57.254 [localhost-startStop-1] INFO org.apache.catalina.startup.HostConfig - Deploying web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/ROOT 19:03:57.307 [localhost-startStop-1] INFO org.apache.jasper.servlet.TldScanner - At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time. 19:03:57.308 [localhost-startStop-1] INFO org.apache.catalina.startup.HostConfig - Deployment of web application directory /usr/local/Cellar/tomcat/8.0.18/libexec/webapps/ROOT has finished in 54 ms 19:03:57.310 [main] INFO org.apache.coyote.http11.Http11NioProtocol - Starting ProtocolHandler ["http-nio-8080"] 19:03:57.314 [main] INFO org.apache.coyote.ajp.AjpNioProtocol - Starting ProtocolHandler ["ajp-nio-8009"] 19:03:57.315 [main] INFO org.apache.catalina.startup.Catalina - Server startup in 1218 ms 

Answer 1

Tomcat log shows that it has one more slf4j-1.7.7.jar in the classpath:

Using CLASSPATH: /usr/local/Cellar/tomcat/8.0.18/libexec/bin/jul-to-slf4j-1.7.7.jar: /usr/local/Cellar/tomcat/8.0.18/libexec/bin/slf4j -api-1.7.7.jar : /usr/local/Cellar/tomcat/8.0.18/libexec/bin/logback-classic-1.1.2.jar: /usr/local/Cellar/tomcat/8.0.18/libexec /bin/logback-core-1.1.2.jar:/usr/local/Cellar/tomcat/8.0.18/libexec/bin/logback-config/:/usr/local/ Basement/cat/8.0.18/libexec/ bin / bootstrap.jar: /usr/local/Cellar/tomcat/8.0.18/libexec/bin/tomcat-juli.jar

When Tomcat was initialized, it initializes jul-to-slf4j and slf4j-api. These banks are loaded by the Classloader loader, which loads the Tomcat banks (sun / misc / Launcher $ AppClassLoader).

When your application tries to access the slf4j-api classes, it uses the WebApplication class loader (org / apache / catalina / loader / WebappClassLoader), which by default looks for the class definition in the application class path to the system class path. Please note that this can be changed: https://tomcat.apache.org/tomcat-8.0-doc/class-loader-howto.html

In Java, a class signature consists of the fully qualified class name and class loader loading the class. If a class is loaded twice from two different class loaders (same hierarchy), there will be two different classes for the JVM. Trying to specify a reference to an object of one class on a variable of another, there will be a trhow exception ClassCastException. Check this blog post (item 5) for more information on this: https://techblug.wordpress.com/2012/03/17/classloader-tips/

So, as far as I know, you can fix this by removing slf4j - *. jar from the Tomcat path, or if you have more than one webapp using the same version of slf4j, you can remove the jars from webapp, so it will use the ones loaded by the Tomcat class loader.